It is often desirable to reinforce rubber articles, such as tires, conveyor belts, power transmission belts, timing belts, and hoses by incorporating steel reinforcing elements therein. Pneumatic vehicle tires are often reinforced with cords prepared from brass coated steel filaments. Such tire cords are generally composed of high carbon steel which is coated with a thin layer of brass. Such a tire cord can be a monofilament, but normally is prepared from several filaments which are stranded together. In most instances, depending upon the type of tire being reinforced, the strands of filaments are further cabled to form the tire cord. The typical steel for tire reinforcement usually contains about 0.65 to 0.75% carbon, 0.5 to 0.7% manganese and 0.15 to 0.3% silicon, with the balance of course being iron.
It is important for the steel alloy utilized in wires for reinforcing elements to exhibit high strength and ductility as well as high fatigue resistance. To obtain this combination of properties the steel wires are generally patented and subsequently cold drawn. The object of parenting is to impart the wire with the ability to withstand a large reduction in area during drawing so that wires having the desired high tensile strength, toughness, and fatigue resistance can be made.
Patenting is normally conducted as a continuous process and typically consists of first heating the alloy to a temperature within the range of about 850.degree. C. to about 1150.degree. C. to form austenite, and then cooling at a rapid rate to a lower temperature at which transformation occurs which changes the microstructure from face centered cubic to body centered cubic and which yields the desired mechanical properties. In many cases, while it is desired to form a single allotrope, a mixture of allotropes having more than one microstructure are in fact produced.
In commercial operations it is desirable for the transformation from a face centered cubic microstructure to a body centered cubic microstructure in the transformation phase of the parenting process to occur as rapidly as possible. The faster the rate of transformation, the less demanding the equipment requirements are at a given throughput. In other words, if more time is required for the transformation to occur, then the length of the transformation zone must be increased to maintain the same level of throughput. It is, of course, also possible to reduce throughputs to accommodate for the low rate of transformation by increasing the residence time in the transformation zone (soak).
In order for rubber articles which are reinforced with steel wire elements to function effectively it is imperative that good adhesion between the rubber and the steel cord be maintained. Thus, generally steel wire reinforcement elements are coated with brass in order to facilitate rubber-metal adhesion.
It is generally agreed by those skilled in the art that adhesion of rubber to brass-plated steel wire is dependent upon a bond between the copper in the brass and sulfur in the rubber. When such brass coated steel reinforcing elements are present in the rubber composition during vulcanization, it is believed that bonds between the rubber and steel reinforcement gradually form due to a chemical reaction between the brass alloy and the rubber at the interface forming a bonding layer. The brass coating also serves an important function as a lubricant during final wet drawing of steel filaments.
Over the years various techniques have been employed for coating steel filaments with brass. For instance, alloy plating has been used to plate steel filaments with brass coatings. Such alloy plating procedures involve the electrodeposition of copper and zinc simultaneously to form a homogeneous brass alloy insitu from a plating solution containing chemically complexing species. This codeposition occurs because the complexing electrolyte provides a cathodic film in which the individual copper and zinc deposition potentials are virtually identical. Alloy plating is typically used to apply alpha-brass coatings containing about 70% copper and 30% zinc. Such coatings provide excellent draw performance and good initial adhesion. However, research in recent years has shown that long-term adhesion during the surface life of a tire depends on more than bulk coating chemistry. More specifically, the nature of the service oxide layer and the chemistry variation (gradient) across the total brass coating have proven to be important.
Sequential plating is a practical technique for applying brass alloys to steel filaments. In such a procedure a copper layer and a zinc layer are sequentially plated onto the steel filament by electrodeposition followed by a thermal diffusion step. For sequential brass plating, copper pyrophosphate and acid zinc sulfate plating solutions are usually employed.
In the standard procedure for plating brass on to steel filaments, the steel filament is first optionally rinsed in hot water at a temperature of greater than about 60.degree. C. The steel filament is then acid pickled in sulfuric acid or hydrochloric acid to remove oxide from the surface. After a water rinse, the filament is coated with copper in a copper pyrophosphate plating solution. The filament is given a negative charge so as to act as a cathode in the plating cell. Copper plates are utilized as the anode. Oxidation of the soluble copper anodes replenishes the electrolyte with copper ions. The copper ions are, of course, reduced at the surface of the steel filament cathode to the metallic state.
The copper plated steel filament is then rinsed and plated with zinc in a zinc plating cell. The copper plated filament is given a negative charge to act as the cathode in the zinc plating cell. A solution of acid zinc sulfate is in the zinc plating cell which is equipped with a soluble zinc anode. During the zinc plating operation, the soluble zinc anode is oxidized to replenish the electrolyte with zinc ions. The zinc ions are reduced at the surface of the copper coated steel filament which acts as a cathode to provide a zinc layer thereon. The acid zinc sulfate bath can also utilize insoluble anodes when accompanied with a suitable zinc ion replenishment system. The filament is then rinsed and heated to a temperature of greater than about 450.degree. C. and preferably within the range of about 500.degree. C. to 550.degree. C. to permit the copper and zinc layers to diffuse thereby forming a brass coating. This is generally accomplished by induction or resistance heating. The filament is then cooled and washed in a dilute phosphoric acid bath at room temperature to remove oxide. The brass coated filament is then rinsed and air dried at a temperature of about 75.degree. C. to about 150.degree. C.